Amplitude adaptive filter

Abstract

A digital filter transfer function is implemented using at least two implementing filters, one of which has a transfer function which guarantees an output which is unconditionally within a predefined output compliance range, and the other of which has a transfer function equal to the difference between the overall desired transfer function and the transfer function of the first implementing filter. Typically, the output samples produced by the two implementing filters are summed to provide an overall output sample. However, if the sum of an output sample from one of the implementing filters and the corresponding output sample from the other implementing filter falls either above or below predetermined threshold values, then the output of the first implementing filter, instead of the summed output, is provided as the sample output of the overall filter. Transient undershoots and overshoots are thus prevented, while the response of the filter is degraded only when necessary to avoid compliance range violation.

I claim: 
     
       1. Digital filter apparatus for use with a plurality of input samples, comprising: a plurality of implementing filters, each coupled to receive said input samples and each of which produces a respective plurality of output samples, each t'th output sample in the plurality produced by any one of said implementing filters corresponding to a respective t'th output sample in the plurality produced by any other of said implementing filters; and   combining circuitry coupled to receive said output samples of said implementing filters, and which produces a plurality of combined output samples, each t'th combined output sample being selectably one of a plurality of functions of the t'th output samples of said implementing filters,   a first one of said functions producing said t'th combined output sample as the sum of the t'th output samples of at least a first and a second one of said implementing filters.   
     
     
       2. Apparatus according to claim 1, wherein said first function produces said t'th combined output sample as the sum of the t'th output samples of all of said implementing filters. 
     
     
       3. Apparatus according to claim 1, wherein a second one of said functions derives said t'th combined output sample from the t'th output sample of only said first implementing filter. 
     
     
       4. Apparatus according to claim 1, wherein a second one of said functions produces said t'th combined output sample as being equal to the t'th output sample of said first implementing filter. 
     
     
       5. Apparatus according to claim 4, for use further with predefined input signal compliance rules and a predefined output signal compliance range, wherein said first implementing filter has a transfer characteristic which precludes said first implementing filter from producing any output samples outside said output signal compliance range as long as said plurality of input samples satisfies said input signal compliance rules. 
     
     
       6. Apparatus according to claim 4, wherein said first implementing filter has a transfer characteristic which can be expressed as a polynomial in z -i  having all non-negative coefficients. 
     
     
       7. Apparatus according to claim 1, wherein a third one of said functions produces said t'th combined output sample as the sum of the t'th output sample of said first implementing filter and a factor p times the t'th output sample of said second implementing filter. 
     
     
       8. Apparatus according to claim 7, wherein p is fixed. 
     
     
       9. Apparatus according to claim 7, wherein p is an integer power of 2. 
     
     
       10. Apparatus according to claim 7, wherein p is selectable among at least two integer powers of 2. 
     
     
       11. Apparatus according to claim 1, wherein said combining circuitry selects said one of said plurality of functions in dependence upon the amplitudes of the t'th output samples of said first and second implementing filters. 
     
     
       12. Apparatus according to claim 1, wherein said combining circuitry selects a second one of said functions different from said first one of said functions if said sum is outside a predetermined range. 
     
     
       13. Apparatus according to claim 12, wherein said second one of said functions produces said t'th combined output sample as being equal to the t'th output sample of said first implementing filter. 
     
     
       14. Apparatus according to claim 1, wherein all of said implementing filters exhibit the same group delay. 
     
     
       15. Apparatus according to claim 14, wherein said group delay is constant. 
     
     
       16. Apparatus according to claim 1, wherein all of said implementing filters are of the continuous stream type. 
     
     
       17. Digital filter apparatus for use with a plurality of input samples, comprising: a first filter that produces a first plurality of output samples in accordance with a first transfer function of said input samples;   a second filter that produces a second plurality of output samples in accordance with a second transfer function of said input samples, each sample in said second plurality of output samples corresponding to a respective sample in said first plurality of output samples; and   means for providing a plurality of filter output samples, each sample in said plurality of filter output samples being equal to a sum of a respective sample in said first plurality of output samples and the corresponding sample in said second plurality of output samples if said sum is within a predetermined range, and being equal to said respective sample in said first plurality of output samples if said sum is outside said predetermined range.   
     
     
       18. Digital filter apparatus for use with an input sample stream, comprising: a first filter having an input coupled to receive said input sample stream, said first filter producing a first output sample stream having samples us t  ;   a second filter having an input coupled to receive said input sample stream, said second filter producing a second output sample stream having samples vs t , each sample in said second output sample stream corresponding to a respective sample in said first output sample stream; and   output providing means for producing a filter output sample stream, each t'th sample of which corresponds to a respective sample in said first output sample stream and to a respective sample in said second output sample stream, and each t'th sample of which is either us t  or us t  +vs t , selectably in dependence upon said corresponding samples us t  and vs t  in said first and second output sample streams.   
     
     
       19. Apparatus according to claim 18, wherein said first filter has a transfer function defined by ##EQU8## where N u  >0 and at least one of said u i  is non-zero; and wherein said second filter has a transfer function defined by ##EQU9## where N v  >0 and at least one of said v i  is non-zero. 
     
     
       20. Apparatus according to claim 19, wherein said first and second filters exhibit equal group delay. 
     
     
       21. Apparatus according to claim 20, wherein said group delay is constant. 
     
     
       22. Apparatus according to claim 19, wherein all of said u i  are non-negative. 
     
     
       23. Apparatus according to claim 22, wherein at least one of the coefficients in z -i  of a combined transfer function defined by   S(z)=U(z)+V(z)     is negative.   
     
     
       24. Apparatus according to claim 19, wherein said first and second filters exhibit equal group delay, wherein all of said u i  are non-negative,   wherein at least one of the coefficients in z -i  of a combined transfer function defined by   S(z)=U(z)+V(z)     is negative,     and wherein said output providing means produces each t'th sample of said filter output stream as us t  +vs t   if us t  +vs t  is within a predetermined range, and as us t  if us t  +vs t  is outside said predetermined range.   
     
     
       25. Apparatus according to claim 19, wherein said first filter comprises: a delay line having an input port coupled to receive said input sample stream and having a tap corresponding to each of said u i  which is non-zero and a tap corresponding to each of said v i  which is non-zero; and   a first summer having an input port for each of said u i  which is non-zero, each i'th one of said input ports of said first summer being coupled to receive the i'th one of said taps weighted by the respective u i , said first summer having an output coupled to provide said first output sample stream,   and wherein said second filter comprises said delay line; and   a second summer having an input port for each of said v i  which is non-zero, each j'th one of said input ports of said second summer being coupled to receive the j'th one of said taps weighted by the respective v j , said second summer having an output coupled to provide said second output sample stream.   
     
     
       26. Digital filter apparatus for use with an input sample bus, comprising: a first filter having an input coupled to said input sample bus and further having an output, said first filter having a transfer function defined by ##EQU10## where N>0, all of said u i  are non-negative and at least one of said u i  is also non-zero;   a second filter having an input coupled to said input sample bus and further having an output, said second filter having a transfer function defined by ##EQU11## where at least one of said v i  is non-zero, V(z) being such that at least one coefficient in z -i  of a combined transfer function defined by   S(z)=U(z)+V(z)     is negative, said second filter having the same group delay characteristic as said first filter;     an adder having a first input coupled to said output of said first filter and a second input coupled to said output of said second filter, said adder further having an output;   a threshold detector having an out-of-range output and asserting said out-of-range output if said sum of the values on said output of said first filter and said output of said second filter is outside a predetermined range; and   a data selector having a first data selector input port coupled to said output of said first filter, a second data selector input port coupled to said output of said adder, and a select input coupled to said out-of-range output of said threshold detector, said data selector further having a data selector output and selecting said first data selector input port to said data selector output if said select input is asserted, and selecting said second data selector input port to said data selector output if said select input is unasserted.   
     
     
       27. Apparatus according to claim 26, wherein said first filter comprises: a delay line having a plurality of taps including taps corresponding to each of said coefficients u i  which is non-zero; and   a summer having a plurality of input ports coupled to respective ones of said taps with weights defined by respective ones of said u i  's,   and wherein said second filter comprises:   said delay line, said taps of said delay line also including taps corresponding to each of said coefficients v i  which is non-zero; and   a summer having a plurality of input ports coupled to respective ones of said taps with weights defined by respective ones of said v i  's.   
     
     
       28. Apparatus according to claim 26, wherein said adder further has an overflow/underflow output and said threshold detector comprises said adder, said out-of-range output of said threshold detector being said overflow/underflow output of said adder. 
     
     
       29. A method for designing a digital filter approximating a desired transfer function defined by ##EQU12## where at least one of said s i  is negative, comprising the steps of: designing a first filter having a transfer function defined by ##EQU13## where all of said u i  are non-negative and at least one of said u i  is also non-zero; designing a second filter having a transfer function defined by   V(z)=C(z)-U(Z); and       designing a circuit which provides to an output, for each given sample output from said first filter, (1) the sum of said given sample output from said first filter and a corresponding sample output from said second filter, if said sum is within a predetermined range, and (2) said given sample output from said first filter if said sum is outside said predetermined range.